A copper foil for printed circuits generally is laminated and bonded to a base of a synthetic resin or the like under high temperature and high pressure, printed with a necessary circuit pattern to form an objective circuit, and then etched to remove unwanted portions. Finally, necessary elements are soldered in place, and in this way various printed circuit boards for electronic devices are fabricated. Qualitative requirements for the copper foil for printed circuits differ with the sides thereof, namely the side to be bonded to the resin base (roughened side or matte side) and the side not to be bonded (shiny side).
Some requirements for the toughened side are:
(1) No possibility of oxidative discoloration during storage; PA1 (2) Adequate resistance to peeling from the base even after high-temperature heating, wet treatment, soldering, and chemical treatment; and PA1 (3) Freedom from so-called stain or defect spots after a lamination operation which may result from foil lamination to the base or from etching. PA1 (1) Good appearance and no oxidative discoloration during storage; PA1 (2) Good solder wettability; PA1 (3) No oxidative discoloration from heating to an elevated temperature; and PA1 (4) Good adhesion to a resist.
Requirements for the shiny side include:
To meet these requirements, many different processes for varied purposes have hitherto been proposed for the treatment of copper foil for printed circuits on each of its toughened and shiny sides. Taking the protection treatment of copper foil against corrosion into special consideration, we previously proposed a process, as Japanese Patent Application Publication No. 33908/1986, which comprises forming a zinc coating film on the shiny side of a copper foil and then forming a chromium corrosion-preventive coating layer of a chromium oxide film. We also taught, as regards the chromium corrosion-preventive coating itself, a coating technique using a mixture of chromium oxide and zinc and/or zinc oxide by electrolytic zinc-chromium treatment (Japanese Patent Application Publication No. 7077/1983). This approach has achieved many fruitful results. Further, Japanese Patent Application Public Disclosure No. 294490/1990 disclosed another process which consists of forming a chromium oxide film by immersion chromate treatment and then forming a mixed film of chromium oxide and zinc and/or zinc oxide by electrolytic zinc-chromium treatment. The process was aimed at preventing the formation of black spots on copper foil stored under hot, humid conditions for long time.
Now, as for the thermal oxidation resistance on the shiny side of copper foil, the requirement has become more stringent within recent years. As one reason therefor, copper foils today are more often exposed to higher temperatures than before due to the advent of novel heat-resisting resins as well as a new fabrication process called "double-layer flexible base process". According to this new process, polyimide varnish is directly applied to copper foil to form a double-layer structure of a polyimide layer and a copper foil layer. Meanwhile, with conventional lamination techniques, the tendency is toward the replacement of the nitrogen atmosphere by air atmosphere for cost reduction of the laminating and curing steps. Both steps of which involve heat treatment. This calls for better thermal oxidation resistance on the shiny side of copper foil. The combined process of zinc plating and chromate treatment currently in practice allows the shiny side to resist thermal oxidation at most, e.g., at 200.degree. C. for about 30 minutes. The foil surface discolors when treated under the higher temperature conditions, e.g., at 240.degree. C. for 30 minutes or at 270.degree. C. for 10 minutes, that will be needed in the years to come. Thus, in order to keep up with the more exacting demand in the future, it is necessary that the copper foil for printed circuits should secure a higher extent of thermal oxidation resistance, or sufficient resistance to remain undiscolored under the higher temperature conditions of 240.degree. C. for 30 minutes or 270.degree. C. for 10 minutes. The improvement should not, of course, have any deleterious effect upon the other properties required of the shiny side, such as solder wettability and adhesion to resist.